Project 1 - Toxicity and Liver Carcinogenicity of 1,4-Dioxane: Single Chemical and Mixtures Studies

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PROJECT SUMMARY 1,4-Dioxane (1,4-DX) is an emerging drinking water contaminant. The potential for 1,4-DX exposure is elevated for people living near Superfund or other types of 1,4-DX release sites. The International Agency for Research on Cancer has classified 1,4-DX as a group 2B carcinogen with the primary organ target being the liver in animal studies. Despite this concern, safety standards for 1,4-DX in drinking water have been slow to develop and vary widely, with the variability being related to the uncertainty associated with its liver carcinogenic potential. Mechanistic studies are urgently needed to (i) understand how 1,4-DX may contribute to liver carcinogenesis by itself or in combination with other co-occurring drinking water contaminants [such as trichloroethylene (TCE) and 1,1-dichloroethane (1,1-DCA)], (ii) determine the exposure concentration range over which these effects occur, and (iii) identify potentially more vulnerable subgroups. Our preliminary studies in mice have revealed molecular targets and pathways potentially involved in 1,4-DX carcinogenicity and set the stage for the proposed studies. These preliminary studies utilized various 1,4-DX concentrations (50, 500 and 5,000 ppm) in drinking water for periods of up to 3 month. These studies revealed mild liver cytotoxicity that is consistent with previous studies. The highest 1,4-DX dose induced assorted molecular changes in the liver including: (i) persistent induction of NRF2 and its target proteins involved in anti-oxidative response (i.e., GCLC, GCLM, HMOX1 and NQO1), (ii) time-dependent induction of CYP2E1 (key oxidative pathway capable of activating endogenous and xenobiotic compounds and a generator of reactive oxygen species), (iii) centrilobular accumulation of the lipid peroxidation by-product 4-HNE, and (iv) elevations in the DNA damage marker γH2AX. Importantly, these 1,4-DX-elicited molecular changes were amplified in a mouse model of systemic glutathione (GSH) deficiency. This project will build upon these intriguing findings and investigate our novel hypothesis predicting that long-term exposure to 1,4-DX causes liver tumorigenesis by disrupting redox homeostasis, thereby potentiating genetic instability. This proposed 1,4-DX mode of action would be of high relevance in assessing carcinogenic effects of co-occurring contaminants that may utilize or modulate overlapping molecular pathways. We propose to (1) delineate the contribution of key redox pathways to 1,4-DX liver carcinogenicity in vivo using transgenic redox mouse models, (2) identify the biological network motifs that predict 1,4-DX-induced liver carcinogenesis and the dose response pattern for perturbation of these networks in vivo, and (3) elucidate the capacity of co-occurring contaminants (TCE and 1,1-DCA) to modify 1,4-DX carcinogenicity in human hepatocyte cells and zebrafish model systems.